A new hybrid composite beam (HCB) has recently been used in the construction of three bridges in Missouri, USA. HCB consists of self-consolidating concrete (SCC) that is poured into classical arch shape and tied at the ends by steel tendons. Both the concrete and the steel are tucked inside a durable fiberglass shell, and the voids are filled with polyiso foam. This paper aims to examine the flexural behavior of an in-service HCB, evaluate the current methodology and assumptions, and propose modifications to that methodology. To achieve these goals, the strains induced in HCB elements due to different loading stages were experimentally measured. Numerical predictions of the strains were performed via the existing methodology, the modified procedure, and a finite element model (FEM) that was constructed using ANSYS V14. The linear FEM predicted the strains with acceptable accuracy. The model clarified that the foam achieves partial composite action between the HCB elements, resulting in a strain incompatibility between them. The current methodology was found to be unable to predict the maximum compressive strain in the concrete arch. The modified procedure is based on the strain compatibility assumption. However, it models the HCBs as curved beam rather than a straight one, using a simplified spring model to represent the beam supports. These modifications achieved significant enhancements in estimating the strains under service loads.